As used herein, the term "arrhythmia" refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF), and bradycardia. As further used herein, the term "therapy"0 refers to any means used by the ICD device to restore normal heart rhythm, such as defibrillation, cardioversion, anfitachycardia pacing and drug infusion. The disclosed invention has application to ICD devices which treat tachyarrhythmias (abnormally high heart rates).
A modem conventional bradycardia pacemaker has a sensing mechanism to enable the device to inhibit pacing when the heart is beating normally. Implantable tachyarrhythmia devices must also sense the heart's intrinsic electrical activity, known as the electrocardiogram (ECG), to determine whether the patient needs treatment. ECGs exhibit highly variable amplitudes and frequencies as the cardiac activity changes from normal sinus rhythms (NSR) to other abnormal rhythms such as ventricular tachycardia and ventricular fibrillation.
U.S. Pat. No. 4,184,493 to Langer et al., which issued on Jan. 22, 1980, and is entitled "Circuit for Monitoring a Heart and for Effecting Cardioversion of a Needy Heart", describes a sensing circuit that automatically adjusts to the amplitude of the heart's electrical signal using an automatic gain control (AGC) system.
Another form of sensing system with AGC is described in U.S. Pat. No. 4,903,699 to Baker et al., which issued on Feb. 27, 1990, and is entitled "Implantable Cardiac Stimulator With Automatic Gain Control". The Baker et al. patent uses a system of comparators and adjustable thresholds to optimally detect the ECG signal.
Both of the foregoing patents describe systems which also filter the ECG signal to remove low frequency noise and artifacts. In addition, high pass filtering is used in these patents to reduce sensing of T waves during normal sinus rhythm (NSR). However, high frequency filtering also attenuates VF signals and makes it difficult to detect the same, especially since VF signals have low amplitudes as compared to signals during NSR.
Another control scheme is disclosed in commonly assigned U.S. Pat. No. 5,395,393, incorporated herein by reference. This patent discloses an ICD in which a different sensing circuit is provided for sensing tachyarrhythmia (including supraventricular tachycardia), ventricular tachycardia or ventricular fibrillation, and for differentiating these conditions from normal sinus rhythm. More specifically, a sensor circuit includes an amplifier, a bandpass filter having a fixed center frequency, a dual dynamic threshold detector and a classifier for detecting tachyarrhythmia using a clustering algorithm. This type of control is commonly referred to as automatic threshold control (ATC) and while it generally works well to differentiate the various types of tachyarrhythmia from random noise, it is generally not effective in detecting tachyarrhythmia in the presence of electromyography signals (EMG) generated by muscles in the chest area, or noise due to standard 50 or 60 Hz power sources.